A B Fig. 6 — The results of TEM analysis of the eutectic microstructures: A — Bright field image; B — SAED results of the different areas in A. ture arrived at 1250ºC, every separated particle of the Laves phase dissolved and formed some separated liquid cells owing to the eutectic reaction between the Laves phase and austenite (or the matrix). At this temperature, the mobility of the liquid was very limited, so every liquid cell stayed where the Laves particle originally was instead of flowing and merging together. In cooling, every separated liquid cell transformed into a solid phase, as shown in Fig. 2C and D. While in specimens experiencing a peak temperature of 1350ºC, the mobility of liquid was greatly promoted by the higher temperature, which means that the liquid could flow in long distances and combine together. In addition, the growth of austenitic grains promoted the impingement of grain boundaries with the liquid regions, so most liquid distributed along grain boundaries or at triple grain junctions, as shown in Fig. 2E and F. Based on the liquation mechanism discussed, the results of hot ductility tests can be explained. In these, the displacement load was applied to the specimens at peak temperatures of 1250º and 1350ºC. At 1250ºC, the limited mobility of liquid retarded wetting grain boundaries, so the grain boundaries’ strength was not impaired. As a result, the specimen exhibited some plasticity at 1250ºC. In specimens experiencing peak temperatures of 1350ºC, some grain boundaries were wet by the liquid and lost strength. When the load was applied, the areas losing grain boundary strength acted as crack initiation, thereby intergranular fracture occurred without any plasticity. Owing to distribution characteristics of the Laves phase, it was mentioned in the “observation of Laves phase in virgin FB2 steel” section, most of the grain boundary area remained free of liquid to maintain some strength, so the specimen at 1350ºC exhibited some load-bearing capacity. The Evolutionary Behavior of Liquid during Cooling It has been mentioned in the section titled “evolution behavior of the Laves phase during the welding thermal cycle,” the eutectic microstructure observed by SEM in Fig. 2C–F is just one of the eutectic constituents, and the other eutectic constituent in the intervals might be etched. Both the morphology and chemical composition of the eutectic constituent were different from those of the Laves phase in virgin FB2 steel, suggesting the eutectic constituent WELDING RESEARCH is a new phase and another type of eutectic reaction might occur during cooling. To clarify the type of the eutectic constituent, thermodynamic software JMatPro® was employed to get all the possible phases in equilibrium condition based on the chemical composition listed in Fig. 3. Although welding is a nonequilibrium process, and the results obtained by JMatPro® were speculative, they can provide some information for reference. The equilibrium phase diagram of the eutectic constituent is shown in Fig. 5. It was evident there are two major phases at room temperature; one is ferrite and the other is the Chi phase. The existence of ferrite in the eutectic constituent can be excluded because ferrite is not corrosion resistant. So the Chi phase is most likely the eutectic constituent. The Chi phase is an intermetallic compound containing primarily Fe, Cr, and Mo. It is a body-centered-cubic phase (-Mn structure) with a lattice parameter of a0 = 0.892 nm (Ref. 19). The Chi phase is often found in austenitic and ferritic stainless steels containing Mo (Refs. 20, 21). Kautz and Gerlach have reported finding the Chi phase as a eutectic constituent in Type 316 stainless steel, which was heated to 1380ºC and then water quenched (Ref. 22). Cieslak and Ritter also found the Chi phase as a eutectic constituent along solidification grain boundaries in CF-8M welds, and they pointed out that the kinetics of the Chi phase formation are greatly enhanced by the presence of Mo, but the mechanism of eutectic reaction was not clarified in his work (Ref. 23). In Cieslak and Ritter’s research, the eutectic Chi phase occurred along the hot crack in welds (Ref. 23). The chemical compositions of the Chi phase obtained by Cieslak (Ref. 23) and Weiss (Ref. 24) are the same as that obtained in the present work to a large extent. All three chemical compositions are summarized in Table 2. Based on morphology and chemical composition already discussed, it is preliminarily speculated that the eutectic constituent observed by SEM is the Chi phase. The speculation is further supported by the results of TEM analysis. The TEM specimen shown in JULY 2016 / WELDING JOURNAL 261-s Table 2 — Chemical Composition of the Chi Phase Obtained by Two Other Authors and in the Present Work (wt%) Material Fe Cr Mo Ni Cieslak (Ref. 23) CF8M austenitic stainless steel 45 26 20 4 Weiss (Ref. 24) 316 austenitic stainless steel 52 21 22 5 Eutectic constituent FB2 martensitic stainless steel 51–54 23–25 18–24 0 in the present work
Welding Journal | July 2016
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